MSc thesis project proposal

[2020] Backing and matching layer fabrication for pressure optimization in monolithically integrated piezoelectric ultrasound transducers for neuromodulation applications

Low intensity focused ultrasound is an emerging non-invasive neuromodulation modality combining high depth of penetration in body tissue with high spatial resolution [1], and its development may lead to non-invasive alternatives to conventional implantable electrodes. Traditionally, neuromodulation experiments with ultrasound have employed single element focused transducers [1] to be able to generate the required acoustic intensities and pulse durations at the focal spot for either stimulation or inhibition of neuronal activity. However, these transducers have bulky form factors (several cm3) and are interfaced with off-the-shelf electronics. As such, they must be mechanically moved (e. g. with motorized stages) to change focal positions and do not translate to wearable form factors for human therapeutics. Two-dimensional (2D) phased arrays are required to create electronically steerable focused ultrasound pressure spots for these applications. The monolithic integration of a 2D array of ultrasound transducers, that is, directly on top of a CMOS chip, is essential to achieve such a phased array design of any considerable size, due to the high density of interconnections required. Close integration also allows parasitic capacitances between electronics and transducers to be reduced substantially.

One of the major challenges in integrating ultrasound transducers into tiny electronic chips [2] is on how to achieve the required acoustic intensities at the focal spot from such miniaturized devices. A key factor in maximizing the focal spot acoustic intensity relies on having a proper acoustic backing layer from the substrate to the piezoelectric transducers, and acoustic matching layer from the piezoelectric transducers to the propagating medium, in this case, body tissue [3]. Without these layers, the transmitted acoustic intensity is highly reduced, and optimization and fabrication of these layers at the microscale of 2D arrays of ultrasound transducers in tiny chips still require extensive research.

This projects targets the design, fabrication and experimental validation of acoustic backing and matching layers to maximize the transmitted acoustic intensities from microscale ultrasound transducer array chips, with the long term goal of achieving wearable ultrasonic neuronal stimulators as a non-invasive and surgery-free alternative to implantable electrodes for the treatment of diseases of the nervous system.

References:

1. Tufail, Y. et al. Transcranial pulsed ultrasound stimulates intact brain circuits. Neuron 66, 681–94 (2010).

2. Shi, C., Costa, T., Elloian, J. & Shepard, K. L., “Monolithic Integration of Micron-scale Piezoelectric Materials with CMOS for Biomedical Applications“ in 2018 IEEE International Electron Devices Meeting (IEDM).

3. Haifeng, W., Ritter, T., Wenwu, C. & Shung, K. K. High frequency properties of passive materials for ultrasonic transducers. IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 48, 78-84, 2001

Assignment

1st part: Literature review of ultrasound transducers backing & matching layers.

2nd part: Design, fabrication and testing of acoustic backing and matching layers to maximize the output acoustic intensity from micron-scale ultrasound transducer arrays.

Requirements

MSc students from Microelectronics, Biomedical Engineering or Mechanical Engineering. Interested students should include their CV, list of courses attended and grades obtained.

Contact

dr. Tiago Costa

Bioelectronics Group

Department of Microelectronics

Last modified: 2021-02-10